1. Field of the Invention
This invention relates generally to the field of semiconductor package wire bonding, and, more particularly, to a novel apparatus and method for producing an ultrasonic vibration mode to improve the bond between a wire and a die or lead.
2. Description of Related Art
Wire bonding today is used throughout the microelectronics industry as a means of interconnecting chips, substrates, and output pins. Automatic ultrasonic gold ball bonding is a high yield interconnect process that uses heat and ultrasonic energy to form a metallurgical bond. Typically, high purity gold wire is used with a ball bond formed at one end and a stitch bond at the other. FIGS. 1a through 1g show the typical sequence of steps involved in forming a gold ball bond. FIG. 1a shows a capillary 10 which is targeted on the bond pad and positioned above a die 12 with a ball 14 formed on the distal end of a wire 16 and pressed against the face of capillary 10. Capillary 10 descends as shown in FIG. 1b, bringing ball 14 into contact with die 12. The inside radius of capillary 10 grips ball 14 in forming the bond. Ultrasonic vibration energy is then applied. The ultrasonic vibration energy is typically produced by piezoelectric transducers. Piezoelectric transducers are well known in the industry and comprise a piezoelectric material, i.e. a material that converts mechanical energy into electrical energy and vice versa. In the case of producing ultrasonic vibration energy, an electric field is applied to a piezoelectric ceramic to stimulate vibration. After ball 14 is bonded to die 12 with the aid of the ultrasonic vibration energy, capillary 10 raises to the loop height position as shown in FIG. 1c. A clamp 18 is then opened and wire 16 is free to feed out of the end of capillary 10. Next, a lead 20 of the device is positioned under capillary 10 and capillary 10 is lowered to the lead. Wire 16 is fed out the end of capillary 10, forming a loop as shown in FIG. 1d. The capillary continues downward and deforms wire 16 against lead 20, producing a wedge-shape bond which has a gradual transition into the wire as shown in FIG. 1e. Ultrasonic vibration energy is once again applied to enhance the bond strength. Capillary 10 then raises off lead 20 as shown in FIG. 1f, leaving a stitch bond. At a pre-set height, clamp 18 is closed while capillary 10 is still rising with the bonding lead. This prevents wire 16 from feeding out capillary 10 and produces an upward force on the bond. The force builds until wire 16 breaks, which it does at the smallest cross section of the bond. Finally, a new ball 14 is formed on the new distal end of wire 16 by employing a hydrogen flame or an electronic spark as shown in FIG. 1g. The process can then be repeated.
Ultrasonic aluminum wire bonding is also a widely used high speed, high throughput interconnect process. In this process, stitch bonds such as described above with reference to FIG. 1f are formed at both ends of the interconnect by a combination of pressure and ultrasonic energy. As the wire softens, freshly exposed metal in the wire comes in contact with the freshly exposed metal on the pad and a metallurgical bond is formed. Aluminum wire is typically doped with silicon (e.g., 1%) to more closely match the hardness of the wire with the bond pad material. Both gold and aluminum wire are used extensively today in packaging, with gold ball to aluminum bond pads being the most common interconnect system.
In conventional wire bonding processes, it is well known by those in the art that bonding strength is enhanced by employing ultrasonic vibration and heat during the bonding procedure and this is typically done. The strength of the bond is only enhanced, however, in the same direction as the ultrasonic vibration being applied. Current processes typically apply only unidirectional vibrations during wire bonding, whereas it would be desirable to enhance the bond strength in all directions. In addition, in order to ensure that integrated circuits are not degraded during the attachment of the bonding wires, it is desirable to conduct the ultrasonic wire bonding at relatively low temperatures. However, the lower the temperature, the more difficult it may be to form a sufficient bond. Therefore there is a continuing need to create better bonds at lower temperatures and at faster rates to increase productivity.
It has been proposed that the application of ultrasonic waves that are circular or elliptical can enhance the bond strength at lower temperatures and with a shorter dwell time, in each of the vibration directions. See e.g., Tsujino, "Ultrasonic wire bonding using high frequency 330, 600 kHz and complex vibration 190 kHz welding systems" (Ultrasonics 34 (1996) 223-228). This strengthening phenomenon has purportedly been achieved by producing the circular or elliptical vibration modes using multiple piezoelectric transducers. It is particularly desirable to generate a circular or elliptical vibration mode for better bond strength in all directions. However, current known methods for producing complex ultrasonic waves using multiple transducers typically employ separate, non-synchronous controls for each transducer, such that error or other difficulties may be introduced by the two separate controls that do not work together and result in a less than ideal higher order wave. In addition, a single transducer with a single control apparatus would be less expensive than two transducers with separate controls. There is a need for production of circular or elliptical ultrasonic vibrations with a mechanism that ensures that the two perpendicular modes needed for circular or elliptical modes are always vibrating synchronously.